Eq. (REF ) establishes a general relation between the thermodynamic forces in the system at constant temperature.
If we choose \((\mu _i)_{P, T}\)  in Eq. (REF ) as the independent driving forces then \(P\)  is fixed.
Conversely, if we use \(P\)  as a driving force, then one of the \((\mu _i)_{P, T}\)  is linearly dependent on the others. The connection between thermodynamic forces (fluxes) avoids problems arising from treating them independently as discussed by Gupta et al. [1]}. Note that the pressure that can be held constant in an experiment is the bulk pressure [2]}, [3]}.
If we impose a bulk pressure gradient, there will be fluid flow.
However, even when the pressure in the bulk of the fluid is constant, the presence of chemical potential gradients can still cause a pressure gradients at an interface.
